1. Field of the Invention
The invention generally relates to a microcoil and a fabrication process of the microcoil, and particularly relates to a high density multi-layer microcoil and a method for fabricating high density multi-layer microcoil.
2. Related Art
The market size of microcoils has become larger and larger. According to conservative market estimate of Nexus, there is a 100 million USD market on basic needs for microcoils, and hard to be estimated if including needs for further applications. The microcoils are so small that conventional coil-winding method has limitation for increasing the density. Now, by applying techniques of MEMS (micro-electro-mechanical-system), the coil size can be in a range of 0.01 to 1 inch. The MEMS is trend of microcoil development.
However, the MEMS generally utilize semi-conductor fabrication processes, which are basically for planar constructions. Therefore, a conventional microcoil is generally of a planar construction that is made by multi-layer depositions of metal and insulation layers, and through photolithography processes.
There have been successful cases of microcoil fabrications through MEMS. For example, as shown in FIG. 1, U.S. Pat. No. 5,852,866, a microcoil includes two stacked coil windings 1a, 1b formed in parallel on a substrate and connected with a through-plating 2; a magnetic core 5 enclosed by a top part 6, a bottom part 8, a surrounding magnetic core area 7 and an insulator 4. Ends of the coil windings 1a, 1b are linked to contact points 3 for use. However, the aforesaid microcoil is a planar construction in which the coil is perpendicular to the substrate that will induce current on the substrate (especially silicon substrate) and waste energy and lower the quality. Therefore, this microcoil is not applicable for high frequency circuits. Moreover, since the axis of the coil winding is perpendicular to the plane of substrate, the coil is hard to be extended in the axial direction. Therefore, it is limited for the number of winding, or the process for multi-layer will be very complicated.
Therefore, this is a requirement for us to acquire a microcoil construction that is parallel to the substrate to avoid current induction, and suitable for high frequency circuit applications.